The invention concerns a steering arrangement with a steering handwheel, which is unrotatably connected with a steering sensor, and with a wheel steered by a steering drive without a mechanically active connection between steering handwheel and steered wheel.
An arrangement of this kind is known from DE 42 07 719 A1. The steering angle, which is transmitted from the steering handwheel via a steering shaft to the steering sensor, is used to control the opening of a hydraulic valve so that a hydraulic motor can steer wheels in one direction or the other. To give the operator a sense of the steering behavior, a counter-action motor is provided, which also acts upon the steering shaft.
The steering arrangement of the present invention is primarily used with driven machines, like, for example, fork lift trucks or forestry machines. In the course of one working day such machines perform many similar movements, for example, when a fork lift truck collects and piles up palettes. In order to optimize the movement processes for the operator ergonomically, so-called mini steering wheels have been introduced. Such a mini steering wheel is known from the Swedish patent SE 466 099. Also in connection with these mini steering wheels a counter-action is desired. However, the advantage of a compact design offered by the mini steering wheel should be maintained, even with the counter-action measures.
The task of the invention is to provide a simple method of managing a counter-action.
In a steering arrangement of the kind mentioned in the introduction, this task is solved in that the steering sensor is made as an electrical machine, working as a generator, for the production of a steering signal and is connected with a switching device, whose load behavior is variable.
This measure provides a relatively simple way of obtaining both the steering signal, that is the desired value specified by the steering handwheel, which must be followed by the steered wheel(s), and a counter-action with which the user xe2x80x9cfeelsxe2x80x9d the resistance to be overcome by the steered wheel. The steering signal is simply produced by an electrical machine working as a generator. By means of the voltage produced by the electrical machine working as a generator and the time it takes to produce the voltage both the steering speed and the steering angle can be calculated. However, the driving of a generator requires a torque, which again depends on the electric load, which must be served by the generator. An illustrative example of such a load is an ohmic resistor, which is connected with the output terminals of the generator. The smaller this resistor is, the larger is the torque required to drive the generator. A change of the electrical resistance thus enables a change of the torque to be produced by the user or operator. Of course there is a number of other opportunities of influencing the xe2x80x9cload behaviorxe2x80x9d. Instead of an ohmic resistor, commonly known electronic or electrical switching elements can be used, which change the current flow between the generator terminals. For example, a keyed switch can be used, whose keying interval can be adjusted. It is also possible to produce a counter-voltage and then feed it to the generator terminals. If, for example, the counter-voltage is larger than the generator output voltage, the user must work against a motor to turn the steering handwheel. This enables a feedback to the driver or user. With this embodiment one single mechanical element is sufficient, namely the electrical machine. An electrical machine of this kind is relatively compact, so that the advantage of a good space utilization is maintained. The combination of signal production with the opportunity of a counter-action gives a very narrow coupling between the two measures, so that major errors caused by different operational behaviors in connection with the production of steering signals and the counter action can be avoided. of course, the size and the performance of the steering handwheel and the electrical machine must be adapted to each other. A small steering handwheel only requires a small machine. The counter-torque or braking-torque can be changed in dependence of various parameters, for example, vehicle speed, vehicle load or time of day.
Preferably, a transmission is arranged between the steering handwheel and the steering sensor. This transmission causes that the electrical machine turns substantially faster than the steering handwheel. For example, a transmission ratio of 5:1 can be assumed. In this case two advantages are obtained. Firstly, a stronger steering signal is achieved, as with a higher speed the generator supplies a stronger voltage signal. Secondly, the counter-action torque of the electrical machine on the steering handwheel is accordingly amplified.
Advantageously, the steering handwheel is made as a mini steering wheel, and the transmission is integrated in the mini steering wheel. With a mini steering wheel no large torques have to be transmitted. The only requirement is that the driver must get a feeling for the steering behavior. In this case it is also sufficient to have an accordingly small or weakly dimensioned gear, which can be incorporated in the mini steering wheel. This means that only very little additional space is required for the gear.
In an alternative embodiment the transmission can be fitted on the outside of the electrical machine. As mentioned above, the transmission is a relatively small component, and the additional space required is hardly mentionable.
Advantageously, the steering handwheel is supported in a housing, in which the electrical machine and at least one part of the switching device influencing the torque are arranged, the switching device having a BUS connection. The BUS connection can, for example, be configured for a CANBUS. Exactly within the vehicle sector the CAN-BUS is very much used as BUS. By means of this CAN-BUS the required information can be transmitted from the steering handwheel to the steering drive and vice versa. It is also relatively simple to introduce additional signals, whose production devices will be described below.
Advantageously, the electrical machine can be driven as a motor. In many cases, electrical machines can be driven both as generators and as motors. When, in the present case, the electrical machine can be driven as a motor, additional opportunities occur, for example, the torque felt by the user can be increased. The motor function can also be used for other purposes.
Preferably, a torque sensor connected with the switching device is arranged on the steered wheel, and the switching device adjusts its load behavior in dependence of the output signal of the torque sensor. Thus, the forces influenced by the steered wheel can be simulated. Particularly, forces can be passed on to the steering handwheel, which act upon the steered wheel from the outside. The operator or driver then gets an even better feeling of the steering behavior of his vehicle. For example, he learns via the steering handwheel, when the steered wheel meets a resistance. By means of the torque sensor it can also be detected when the steered wheel reaches its end stop. This keeps the wear at a minimum or prevents damages.
In an advantageous embodiment the steered wheel has an end-stop sensing device connected with the switching device, and the switching device produces an irregular torque at the electrical machine, when the steered wheel reaches a movement limit. Then the operator will be informed that the steered wheel has reached its end stop, for example in that position the steering wheel vibrates or is exposed to a pulsating torque. Thus, an oversteering of the steering arrangement is prevented. The torque can be produced by a motor or be a varying resistance torque.
Preferably, the steered wheel is provided with a position sensor connected with the switching device, the. switching device returning the steered wheel to a neutral position. This may, for example, happen, when the steering handwheel does not move for a predetermined period. This gives an automatic return of the vehicle to the straight-forward position, whenever required.
In this connection it is advantageous if the switching device triggers the electrical machine synchronously with the steered wheel. Then the user simultaneously gets the information that the steered wheel is moved back to the neutral position. This enables him to interrupt this movement by an operation of the steering handwheel, for example by holding it. If he does not, the steering handwheel will also be in the neutral position at the end of the returning process. If desired, this procedure can be limited to standstill periods of the vehicle.
In a preferred embodiment it is provided that the switching device compares the steering handwheel position with the steered wheel position and adjusts the torque on the steering handwheel in dependence of the difference. Thus, for example, the steering handwheel acceleration can be influenced so that the steering handwheel cannot be turned faster than for the steered wheel to follow. When there is a large deviation between the two positions, for example, the steering handwheel is far ahead of the steered wheel. When then the torque is increased, which the operator must overcome, the steered wheel has the opportunity of catching up again.
Advantageously, the steering handwheel has a marking, and an additional stationary marking is provided in the steering handwheel surroundings, the switching device trigging the electrical machine to work as a motor, in a way that the relative position of the two markings corresponds to the angle position of the steered wheel. For example, when the steering system is used in a fork lift truck, it is important to know in which position the steered wheel is, particularly when the operator comes back and starts the vehicle again after having been away. For this purpose it is known to provide some sort of graphic representation of the wheel positions. However, advantageously, this position statement can also be realized via the steering handwheel, particularly when the steering handwheel is made as a mini steering wheel. As it is possible to act upon and adjust the angle position of the steering handwheel by means of the electrical machine working as a motor, it can be imagined that the position of the steering handwheel is adapted to the position of the steered wheel, each time the vehicle stands still and the steering handwheel is not operated for a predetermined time. For this purpose, the angle position of the steering handwheel needs not be particularly accurate, as the crucial thing is to give the operator a feeling of the position of the steered wheel.